Atomic absorption spectrometers are well known and analyse a sample material by directing a beam of electromagnetic radiation through a sample and then detecting absorption of the beam by the sample and therefore the concentration of the sample.
Atomic absorption spectrometers usually carry a carousel of hollow cathode lamps which are selectively placed into alignment with the optical axis of the instrument to enable electromagnetic radiation of a particularly wavelength to be directed to a sample. The optical path of the instrument generally comprises an array of lenses or mirrors and a sample stage interposed in the optical path through which the electromagnetic radiation passes. Electromagnetic radiation is directed to a monochromater which includes a monochromater mirror for reflecting the radiation to a diffraction grating which reflects the radiation back to the mirror. The mirror reflects the radiation to a detector for analysis. The diffraction grating can be moved under computer control to tune the instrument to the particular wavelength which is emitted by the cathode lamp. In general, the instrument will step through a number of analysis steps, each using a different cathode lamp to provide radiation of different wavelength which passes through the sample. The diffraction grating is moved under computer control so as to reflect that wavelength back to the monochromater mirror for reflection to the detector. The absorption of that wavelength by the sample and therefore the lack of detection of that particular wavelength by the detector indicates that the sample does include atoms of a particular type which absorb that wavelength and therefore constituents of the sample can be identified.
The light which enters the monochromater passes through a vertical slit for receipt by the monochromater mirror and after reflection by the diffraction grating, and the monochromater mirror towards the detector, passes through another vertical slit to be received by the detector.
The cathode lamps which produce the radiation generally comprise a 3 mm diameter source which is directed by the optics of the instrument to focus at the sample to provide a 3 mm diameter source image at the sample. The radiation then passes to the slit in the monochromater and passes through the slit into the monochromater. Typically the slit is about 0.25 mm wide. An image of the slit or, in other words, the image which is received by the detector when focused at the source is therefore a vertical slit of about 0.25 mm width. Radiation produced by the cathode tube and which falls outside the perimeter of the slit is therefore not received by the photodetector of the spectrometer, and therefore plays no function in analysis of the sample material. Thus, a significant amount of 3 mm diameter spot of light at the sample stage is lost.
In order to provide sample material for analysis, the spectrometer includes a burner which produces a flame to ionize sample material which is introduced into the flame. The ionized sample material in general is carried up with the flame in the burner and the radiation from the cathode tube is focused in the flame at the sample position so that the radiation, more likely than not, will pass through atoms of the sample material and be absorbed. By detecting absorption of the radiation, the constituents of the sample material can be measured as is described above.
In general, because the sample is ionized in a flame, the atoms of the sample will move upwardly with the flame and will pass through the image of the slit of the monochromater at the sample stage therefore falling within the path of the radiation which passes through the source image and therefore through the slit in the monochromater for detection.
However, if other methods of producing sample material are utilized, the likelihood of the radiation from the source passing through sample atoms can be much less. For example, if sample is produced in a graphite furnace rather than a flame, there is a significant possibility that sample atoms will not locate in the source image of the slit at the sample location and therefore will not fall within the path of the radiation which is actually detected by the detector. Thus, there is a possibility that sample atoms and therefore the true constituent nature of the sample material will not be determined.
Graphite furnaces generally comprise a graphite tube of circular cross-section which is located at the sample stage. The graphite tube is open at both ends and the radiation passes through the tube. High electric current is supplied across the graphite tube to heat the graphite tube and therefore atoms of sample material which is deposited in the tube. In general, the sample material is deposited in the tube by a very thin needle which passes through an aperture or bore in the instrument and through an aperture or bore in the graphite tube. With conventional instruments, considerable skill is required in order to deposit the sample material at a correct location so that when the graphite furnace is energized, sample atoms will rise in the graphite furnace through the source image of the slit and therefore in the path of radiation which is actually detected by the detector.
If the sample material is not deposited centrally in the graphite furnace, but slightly to one side, the possibility exists that when the graphite furnace is heated, the sample atoms will travel vertically upwards and miss the source image of the slit and therefore not fall within the path of radiation which is actually detected by the detector. Thus, those sample atoms will not be detected, thereby resulting in improper or, in fact, no analysis result of the sample material.
The slit in the monochromater which allows the radiation to pass into the monochromater is required in order to block out unwanted wavelengths and also to prevent the detector from detecting extraneous radiation which may completely smother wavelengths which the detector desires to detect. In particular, with a graphite furnace, since the graphite furnace is heated to high temperature and glows white hot, the slit is required to be positioned so that it does not allow imaging of radiation produced from the graphite furnace itself onto the detector, which would otherwise saturate the detector and prevent proper analysis of radiation which passes through the sample and which is produced by the cathode tube. To prevent extraneous radiation from being detected by the detector, the size of the slit is changed depending on the wavelength being detected and also, in some instances, the slit is masked to reduce the length of the slit to ensure that radiation which is produced by the graphite furnace itself is not received by the detector.
Thus, the fact that the masked slit only allows part of the radiation which passes through the sample to enter the monochromater and the orientation of the slit can therefore greatly reduce the sensitivity of the spectrometer.